The present invention relates to radiating blade antennas and more particularly to such antennas for use on aircraft and the like.
Conventional resonating monopole blade antennas for aircraft are known in a variety of designs and functions.
One class of such antennas includes a base plate, the latter serving as part of a ground plane structure. One such antenna 50 schematically shown in FIG. 4 includes the base plate 52, an input connector 54 mounted through the plate 52 through which extend ternfinal lug 56 and wire 58. A metal folded blade radiator 51 mounts vertically and above plate 52 on dielectric insulator 53. Lumped compensation members can be included in such an integrated matching stub 55 bonded to one face of blade 51 and having its outer end shorted to the blade face by a solder point and its other end connected to wire 58. The dimensions of the radiator legs and the stub are determined for the values they provide at the operating wavelength and the electrical values provided in accordance with well-known design principles. Ideally, coaxial stub length is selected to equal 1/4 of the wavelength at the center frequency (taking into account the phase velocity of the stub material), in order to broaden the operating frequency band by effectively neutralizing bandlimiting radiation reactance. The impedance of said coaxial stub is chosen so as to maximize the effective antenna bandwidth in direct trade-off for a marginally degraded VSWR over the operating frequency band. The characteristic VSWR response is that of a second order Chebychev polynomial or Maximally Flat/Butterworth function.
Another standard radiating antenna is schematically shown in FIG. 5 that includes a base plate 62 and an input connector 64 mounted through plate 62. The vertically oriented metallic radiator blade 61 mounts above plate 62 on dielectric mounts or "standoffs" 68. The bare end of wire 66 is soldered to blade 61. In this configuration, no lumped elements are provided, except for the connecting wire 66, which has some value of intrinsic series inductance. Equivalent modeling of the blade will result in an equivalent lumped L.C.R. circuit topology and is embodied as part of the design characteristics.
The chief motivation for implementing such an antenna configuration of FIG. 5, instead of that shown on FIG. 4, is to exploit the condition that an antenna radiator taller than 1/4 wavelength will have a different electromagnetic radiation pattern and gain value. With increased physical and electrical height, the radiated, or received, RF energy will tend to be biased at lower angles of elevation towards the horizon, with an increased gain. The inherent difficulty associated with this scheme is that the radiator is not a purely resonant structure at mid-band frequencies. Consequently, companion quarter wave stub matching methods are no longer appropriate for they would tend to interfere with the non-resonant radiator operation characteristics. This disadvantage is overcome by placing a rectangular slot in the base of the radiator so as to allow for the simultaneous combination of additional values of lumped series inductance in the form of wire 66 in FIG. 5, while reducing the undesirable distributed capacitance between the radiator and the base plate, near the connector feed point region.